In recent years, there has been a strong demand for compact designs, high efficiency, and high output for electrical devices equipped with an electromagnetic valve, a motor, or a power supply circuit. With these electrical devices, using high frequencies as the operating frequency range is effective. Thus, higher frequencies are being used more and more, e.g., from hundreds of Hz to several kHz for electromagnetic valves, motors, and the like, and from tens of kHz to hundreds of kHz for power supply circuits.
Electrical devices such as electromagnetic valves and motors have been operated primarily with frequencies of no more than hundreds of Hz, and used so-called electromagnetic steel plates as the material for the iron core due to the low iron loss of this material. The iron loss in the core material can be broadly divided into hysteresis loss and eddy current loss. The surfaces of thin plates of an iron-silicon alloy, which has a relatively low coercive force, are insulated, and the plates are stacked to form the electromagnetic steel plate described above. It is known that low hysteresis loss is provided with this structure. While eddy current loss is proportional to the square of the operating frequency, hysteresis loss is linear to the operating frequency. Thus, if the operating frequency is no more than hundreds of Hz, hysteresis loss is dominant. Thus, in this frequency range, the use of electromagnetic steel plates, which have low hysteresis loss, is especially effective.
However, since eddy current loss becomes dominant when the operating frequency is more than 1 kHz, the iron core must be made from a material other than electromagnetic steel plates. Powder cores and soft ferrite cores, which have relatively low eddy current loss properties, are effective in these cases. Powder cores are made using a soft magnetic material in powder form, e.g., iron, an iron-silicon alloy, a Sendust alloy, a permalloy, or an iron-based amorphous alloy. More specifically, a binder member having superior insulation properties is mixed with the soft magnetic material or the surfaces of the powder are insulated, and the resulting powder is compacted to form the powder core.
Soft ferrite cores are known to be especially effective as a material with low eddy current loss since the material itself has a high electrical resistance. However, the low saturation flux density resulting from the use of soft ferrite makes high outputs difficult to obtain. In this regard, powder cores are effective since their main component is soft magnetic material, which has a high saturation flux density.
Also, the making of powder cores involves compacting, and this introduces distortion in the powder due to deformation. This increases coercive force and leads to high hysteresis loss in the powder core. Thus, when a powder core is to be used as a core material, an operation must be performed to remove distortions after the shaped body has been pressed.
One effective way to remove distortions is to perform thermal annealing on the shaped body. Distortions can be removed more effectively and hysteresis loss can be reduced by using higher temperatures for the heat treatment. However, if the heat treatment temperature is set too high, the insulative binder member or the insulative coating in the soft magnetic material can break down or degrade, leading to higher eddy current loss. Thus, heat treatment can be performed only within a temperature range that does not lead to this problem. As a result, the improvement of the heat resistance of the insulative binder member or the insulative coating of the soft magnetic material is an important factor in reducing iron loss in the powder core.
In a representative example of a conventional powder core, approximately 0.05 percent by mass to 0.5 percent by mass of a resin member was added to a pure iron powder formed with a phosphate coating serving as an insulative coating. This was then heated and shaped, and thermal annealing was performed to remove distortion. In this case, the heat treatment temperature was approximately 200 deg C. to 500 deg C., the thermal decomposition temperature of the insulative coating. Because of the low heat treatment temperature, however, adequate distortion removal could not be obtained.
Japanese Laid-Open Patent Publication Number 2003-303711 discloses an iron base powder and powder core using the same that includes a heat-resistant insulation coating wherein the insulation is not destroyed when annealing is performed to reduce hysteresis loss (Patent Document 1). With the iron base powder disclosed in Patent Document 1, the surfaces of a powder having iron as its main component are covered with a coating containing silicone resin and pigment. It would be preferable for a coating containing a material such as a silicon compound to serve as a lower layer of the coating containing silicone resin and pigment. For the pigment, a powder with a D50 rating and having a mean particle diameter of 40 microns would be preferable.
[Patent Document 1] Japanese Laid-Open Patent Publication Number 2003-303711